lower bound = 20

C(27,7,2) = 20

Method of Construction: communicated by d. todorov

Lower Bound: Schonheim

  1  2  3  4 15 17 24
  5  6  7  8 17 21 25
  9 10 11 12 17 22 26
 13 14 15 16 17 23 27
  1  2  6 11 16 18 25
  2  5 12 15 18 21 24
  3  8  9 14 18 22 27
  4  7 10 13 18 23 26
  1  2  8 10 15 19 26
  2  7  9 16 19 22 24
  3  6 12 13 19 23 25
  4  5 11 14 19 21 27
  1  2  7 12 14 20 27
  2  8 11 13 20 23 24
  3  5 10 16 20 21 26
  4  6  9 15 20 22 25
  1  5  9 13 21 22 23
  6 10 14 24 25 26 27
  3  4  7  8 11 12 16
  7 11 15 17 18 19 20